As a fuel-saving measure, internal combustion engines today are operated with a lean combustion mixture. This results in nitrogen oxides NOx being unable to react completely in a conventional catalytic converter, because the reducing components required for the reaction are no longer present in the exhaust gas in an adequate concentration. This is the reason for using NOx storage catalysts, which are capable of storing unconverted NOx. They are regenerated intermittently by supplying reducing exhaust gas components.
Conventional commercial fuels contain small amounts of sulfur compounds, which release sulfur in the form of sulfur oxides during combustion of the fuel. SO2 in particular is stored in the NOx storage catalyst in competition with nitrogen oxides, thereby decreasing the catalyst ability to absorb nitrogen oxides. There is an increasing accumulation of sulfur oxides in the NOx storage catalyst and thus a decreased storage capacity of the latter because nitrogen oxides are released in the intermittent regeneration of the NOx storage catalyst and are ideally converted to nitrogen, and yet the incorporated SO2 remains in the NOx storage catalyst under the conditions prevailing during regeneration. To overcome this problem, a sulfur storage device may also be connected upstream from the NOx storage catalyst to absorb the sulfur compounds present in the exhaust gas before reaching the NOx storage catalyst.
In both cases, desulfurization must be performed intermittently when the storage capacity of the NOx storage catalyst or the sulfur storage device drops below a certain limit. German Published Patent Application No. 199 10 503 describes that an elevated temperature of 550° C. to 700° C. may be induced in the NOx storage catalyst or the sulfur storage device to perform desulfurization, and the combustion mixture may be established at a lambda value of <1.
It is a problem to determine a point in time when storage capacity of the NOx storage catalyst and/or the sulfur storage device has dropped below a certain limit and desulfurization must be initiated. In German Published Patent Application No. 199 10 503, desulfurization is performed periodically on the basis of characteristic data obtained in preliminary experiments. However, flexible control is not possible in this way.